Electron-discharge apparatus



Aug'. a 192e. 17,594,630

M. MORRISON Y ELECTRON DISCHARGE APPARATUS Filed Janf 10. 1924 Patented Aug., 3, 1925.

TE STATES IVIONTFORD MORRISON, CHICAGO, ILLINOIS.

ELECTRON-IDISCHARGE APPARATUS.

Application led January 10,l 1924. Serial No. 685,409.

My present invention relates to devices which operate with the assistance of incandescent cathodes and has particular reference to such devices Where the discharge produced is subie-:ft to variations in behavior dueto changes in gas ionization. Within the devices, differences in specific electron emissivity and instability' of spontaneous electron liberation.

among the objects of my invention are; first, to provide means for stabilizing the electron discharge ofthe apparatus when the source of electric current for heating the aforesaid incandescent cathodes is subject to variation; second, to provide an improved constancy of conductivity Within the device when the said device is subject to gas ionization; third, to provide an improved conancy of the electric conductvity in the dewhen the specific einissivity of the i n caiidescent cathodes is subject to variation: fourth, to provide an improved constancy of conductivity of the device when spontaneous ionization may be present or subject to variation.

Other and further object-s will be in part pointed out and inpart obvious upon read* ing the speciiications andinspecting the drawinmbut the novel scope of my invention will be pointed outvvith certain particularities in the appended claims. 4

For a clear understanding of my invention reference may be had to the folloW- ing description bearing connection with the accompanying drawing in which the figure is a general and diagrammatic view of one einoodiment of my invention.

It is admitted that the art of controlling` electron discharge apparatus by means of the discharge current through the tube is notneiv. Such apparatus has been in eX- tensive use in connection With discharge tubes operating With pure gas ionization and in those in which only non-gas ionization is alleged to occur.

The objects of stabilizers of the prior art for electron discharge tubes have been particularly directed toward the stabilization of X-ray tubes when the operation is supposed to take place independently of gaseous ionization, and their principal object-s have been to provide means. for automatically regulating the elect on emission, particular-Y ly when the source of heating current for the incandescent cathode varies in voltage due inherently to line fluctuations and it Was thought that this was the principalV source of operative erraticalness in these tubes. However, I Vhave discovered-that there are four sources of'variation in the electrical conductivity of these tubes and that the principal difiiculty does not originate from a variation in filament excitation voltage but from variation in the aggregate electron emission from the four sources.

The four sources which I have discovered are variations in the temperature of the cathode, variations inthe specific Velectron einissivity of the hot cathode itself, variations in the gas `pressure Within the tube which not only affect the gaseous ionization but also the electron emission of the filament itself as `Well-as do they even change the spectrum of the target due to interference vvith the velocity distribution of. the electron stream, and also variations in spontaneous ionization which is the pulling out of electrons themselves from parts ofthe cathode on Which the electrostatic stresses are concentrated, due to points, sharp edges andthe like.. Y

It will be appreciated by those familiar` i with the art Ythat in vievv of this eXplanation it is erroneous to assume that the sta.- bilization of any one of these phenomena Will affect stabilization of the discharge current through the tube itself; and further, that if the discharge current is to be stabilized the aggregate electron liberation must be made constant as a Whole and this may be done by selecting one ofk thepredominating factors of the aforementioned group which may be controlled external of the tube and construct a device which Will add to or subtract from the value of this selected factor in such a Way as to maintain the aggregate at a substantially constant value. Y

Means and methods for doing this are the substance of this invention.v

In all the devices of prior art, some sort of a relay has been used to accomplish their result. In the first case 'on record, a simple relay element in which the armature is actuated by the discharge current through the tube is made to operatel an osmosis regulator which lowers or raises the conductivity of the said tube; In other developments of the relay element above described instead of being actuated by intermittent currents, is made to vibrate synchronously with thecalteration of the line voltage and this vibratthe line voltage. The performance imposed upon this instrument and the limits of yariations in its behavior result in this being almost impractical as an loperative device and itsv use in the art for this reason has been largely abandoned. l In my device l employ only rotating machinery, obviating to the use of uncertain vibrating parts and, therefore, produce a structure Which is in considerable advance in the art Referring to the figure, l is a source of alternating potential which by means of leads 2 and 3 Vsupply a primary 4 of the step-up transformei' 5 With exciting potential. This primary circuit may be supplied with the sivitch G and means 7 of varying the potential impressed upon by the primary. S is the high potentialwvinding of the transformer 5, which by means of leads 9 and 10 admit said high potential to the alternating current terminals 11 and 12 of a rectifier 13, preferably of the kind set forth in Amy United States Letters Patent 1,466,541; 14 and 15 arethe direct current terminals of the said rectifier. The terminal 14 conducts current by means or lead 16 to the anode terminal 17 of an X-ray tube V13 preferably enclosed Within the shield 19, as set forth in my application, Serial No. 633,712, filed April 21, 1923. Direct current terminal 15 byA means of lead 2O conducts negative potential from rectifier 13 to my stabilizing apparatus. Rectifier 13 is driven synchronously With source of alternating current potential 1, by mechanical connection or by electrical means such as a synchronous motor 21 operating from source of alternating current 1 through leads 22 and 23, the circuit of which may be supplied With the switch 24. Of course, in practice the synchronous motor 21 would be connected mechanically to rectifier 13, but this connection is omitted in the diagram for 'sake of clearness. y

The source of alternating potential 1 also supplies insulation transformer 25 by means of leads 2,6 and 27 Withcthe operating current for said transformer, the primary circuit of Whichmay be supplied with switch 28. AThe function of the insulation transformer 25 is to provide electrical insulation between ther corona-proof housing 29 and source of alternating potential 1. t is not a requisite ofthe invention that alternating source of potential 1 be insulated by means of the transformer nor that the transformer be employed at all. It only happens that in the illustrated embodiineiitof this invention it is convenient and this fact fWill be more fully pointed out and apparent upon reading the description of the contents of the corona-proof housing 29.

` The secondary circuit 30 of transformer 25 is supplied with energy lby induction from primary 31. insulation transformer 25 supplies operating current for the stator of an induction motor Whose rotor is represented by 33. The rotor 39 is in mechanical connection with an exciter-generator 34. 35 is the exciter armature supplying excitation forits shunt field 36 and for the shunt field 37 on the generator 39. In some cases I prefer to shunt the said fields With the con denser 33 for `electrical protection of the shunt field insulation against oscillatory disturbances. The generator 39 which is used to supply exciting current for the fila ment of the X-ray tube 18, is so designed that its magnetic circuit operates considerably below the linee of the inagnetizing current, providing the said generator With an op rating voltage which is sensitive to Varation in the field-excitation current.

For an Xfray tube filament which Would require 50 Watts for full load excitation, the required generator frame for 39 vwould bc selected such as would have a normal Watt output of say vvice 50 Watts or 100 Watts and then the field excitation reduced 50% giving about the desired saturation characteristics. Though in reality the frame construction of the field circuit for this generator is not thus established on account of the large number of turns rused on the series field 40. This series field V40 is operated in series With the X-ray tuber discharge current and unlike most series fields has a large number of turns, usually many times more than the field 37 because of the small value of the discharge in the tube. The field 40may be shunted by a condenser 41 and a variable shunt 42.

The particular attention at this point is directed to the use of the condenser 41, as a part of the three circuits 40, 41 and 42. ll/ fatheinatically it can be shownv that for any electricr circuit having a pulsating direct current upon being conducted through the divided circuit divides itself in such a Way that at the end of any cycle, the total current Which passes through any particular circuit depends solely upon the resistance of the circuits and entirely independent of the inductancc or capacity. This means that the highly inductive character of .field 40 does not influence the total field current therethrough, and that the condenser 41 has no effect on the total current except to the ex- 1 tent'of its leakage, and also resistan-ce 42 acts as a simple rohmic shunt in conjunction with the field 40. The further act-ion of the condenser 41 is to prevent high instantaneous rise of voltage across the field 40 and further tends to smoothen out the current wave-form. which represents the discharge current through the tube.

The brushes 43 and 44 of generator 39 may be shunted by condenser 45 and a resistance 46 which has at its middle point a connection with the variable shunt 42. The filament 47 of the -ray tube 18 is connected in parallel with the brushes 43 and 44 of the generator 39. The field 40 is connected diderentially so that with an increase of current in lead 20, considerable demagnetization of the field current of generator 39 takes place.

The operation of my device is as follows: Source of potential 1 is made to function, switch 28 is closed which starts and operates the rotor 33 at its proper speed. The generator 39 supplies current to the cathode 47, brings its temperature up to an electron emitting temperature, switch 24 is closed, rectifier 13 operates at its proper speed and phase relation with the system and in proper polarity. lThe voltage on primary 4 is ad justed by means of 7 to its proper value, the switch G is closed at which event rectifier 13 begins to supply high potential tothe X-ray tube 18, which in turn allows currend to flow from the filament 47 to the anode 48. The generator 39 being very sensitive to difference in total amperes field turns tends to have its field weaken because of the X-ray tube discharge current acting differentially upon field 40. rllhis field weakening process continues until a balance is assumed in the electrical circuits. This condition of balance will be called a normal 4operating condition, that is to say, the circuit can so adjust itself that when this balance occurs, which is, of course, all within a very small fraction of a second, the apparatus is operating as desired.

First, suppose the voltage of alternating source of potential 1 should drop. In the absence of my device, through the normal course of events filament 47 would have its temperature lowered which would mean less discharge current. rfhis is that which is to be avoided. ln case of my device being in the circuit, the effect of the drop in voltage at 1 would not effect materially the speed of rotor 38 as the speed of an induction motor is not influenced materially by a-decrease in excitation voltage. But, if it is assumed that the speed of motor 33 drops in value, the generator 39 would also have a drop in voltage. This in turn would tend to reduce the temperature of filament 47 which has the effect of decreasing the discharge through the X-ray tube thus loweringv some of the demagnetizing edect of said current in field 40, resulting inv a tendency to maintain a high voltage in the generator 39 and to tend to keep it at its normal value.

It will be appreciated that the discharge from cathode 47 to anode 48 is very sensitive to small differences in excitation currents in cathode 47 which materially assists in the balancing operation of the two fields at generator 89.

Should the voltage of alternatingpotential 1 rise in value, the reverse of the operation advanced above would occur. be said here that the major portion of the line variations are eliminated by the inherent constant speed frequency characteristics yof the driving motor 33, and this characteristic in practice is more effective and useful in the elimination of line lvoltage variation than the theory involved in the aforementioned discussion. However, both are present. ln some cases l may use a synchronous rotor instead of the plain squirrel cage rotor.

The most important use of my device, however, is not for the elimination of line variation in exciting current of the filament of an X-ray tube but the elimination of erratic behavior in Xeray Vtubes employing hot cathodes. Although these tubes are alleged to be independent of gas ionizaion, exper- It may Y ience in the art shows that there is an ap-l preciable amount of gas nearly always present and that the actual pressure of this gas can vary from instant to instant. During normal operation there may occur gas ionization at'times Vand sufficient to injure the tube if not promptly Vtaken care of, when it occurs in over-abundance. The eect of this gas ionization is to increase the conductivity of the tube, that is, with a constant filament temperature the conductivity may rise or lower of its own accord. Y

Assuming that my device is in operation as heretofore described and in the above con# dition and that the temperature of the filament 47 does not vary but that due to the presence of more than the normal amount of gas in the X-ray tube, more discharge current is forced through the tube. greater demagnetizing effect is set up by field 40 and the voltage ofthe generator 39 is materially decreased, lowering the temper ature of the filament 47 or even to a point where it fails to emit electrons. The tube may even work for a few cycles upon a gas ionization. Should the conductivity of the said X-ray tube be decreased due to a decrease in the pressure of the gas in the tube over and above the normal amount'the reverse of the above described action takes place. 1f the specific emissivity of the filament 47 changes for any reason whatever, the same operation occurs as if the tube had this gas pressure changed as it isimmaterial whether the gas pressure changes or the specific emissivity changes. rlhe result vis a change in the conductivity of the tube and my device functions alike in both cases. The same action takes place in the presence of variable spontaneous ionization.

' it will be appreciated bv those skilled in the art to which my invention appertains that various modifications of my invention may be made Without departing from the spirit of my invention such as for example,the use of my device Without rectifier 13 and with a different type of X-ray tube and connection With various-apparatus such as is common in the art or Which may develop( in the future.

A`What I claim is:

l. In an electron discharge apparatus, an electron discharge tube having main-electrodes, one of which is a therinionic cathode, amain circuit for supplying main-electrode current, an independent source oi:` heating current for said cathode, said source coinprising an` electric generator provided With shunt'and differentially compounded fields electrically associated With said main-electrode circuit, and means `for adjusting said association With Vthe said fields, whereby the range. of stable main electrode current may be lined at a predetermined mean-value.

2. In an electron discharge apparatus, an electron discharge tube having mainelectrodes, one yof which is a thermionic cathode, a main circuit for supplying main electrode current, independent source ot heating current for said cathode, said source comprising an electric generator provided with shunt and diii'e'rentially compounded fields, said shunt lield provided with a source of excita.- tien independent of the voltage of said generator, and said differential field electrically associated with said main electrode circuit, and means for adjusting'the compounding of said fieldsl whereby the range of stable main-electrode currents may be adjusted to a desired mean value.

3. In an'electron discharge apparatus, an electron discharge tube having` main electrodes, one ot which is a thermionic cathode, a main circuit for supplying main-electrode current, an independent source of heating current for said cathode, said source comprising an electric generator provided With shunt and ditierentially compounded fields electrically associated with said maineled trode circuit, and means for adjusting said field relations whereby the range of stable main-electrode current may be fixed at a predetermined value.

4. In an electronzdischarge apparatus, an electron discharge tube having main-electrodes, one of which is a thermionic cathode, a main-circuit for supplying main-electrode current, an independent source of heating current tor said cathode, said source comprising an electric generator provided With shunt and differentially compounded iields, said shunt provided With a source oit excitation independent ot the voltage or' said generator, said diierential field electrically associated Wth main-electrode circuit, and means ror'adjusting the said association, whereby the range of stable main-electrode current may be iixed at a predetermined value.

In Witness whereof, I have hereunto set my hand this 8th day of January, 1924.

MONTF ORD MORRISGN. 

